As is well known in the art, the vortex flow meter utilizes the fact that, when a vortex generator is arranged in a fluid flow, within a predetermined Reynolds number range, the number of Karman vortexes generated from the vortex generator per unit time (vortex frequency) is in proportion to the flow rate regardless of whether the fluid is gas or liquid; this constant of proportionality is called Strouhal number. As a detection sensor for the vortex, there are exemplified a thermal sensor, a distortion sensor, a photo sensor, a pressure sensor, and an ultrasonic sensor; these are capable of detecting thermal change, lift change, etc. due to a vortex. The vortex flow meter is a simple flow meter capable of measuring flow rate without being affected by the physical properties of the fluid to be measured, and is widely used for flow rate measurement of gas and fluid. In a vortex flow meter, a Karman vortex signal from a detection sensor is triggered by a trigger circuit to obtain a flow rate signal pulse.
In a flow rate measurement mechanism for a compressible fluid, even when the flow is stopped by closing a cutoff valve provided on the downstream or the upstream of a flow meter, the compressible fluid such as gas is compressed/expanded within a closed tubular path to thereby cause pulsation in the fluid. A flow rate detecting portion detects movement of the fluid to cause a signal transmitting portion connected to this flow rate detecting portion to transmit a signal, disadvantageously resulting in a state as if flow rate measurement has been performed. Apart from the noise signal due to this fluid rocking (pulsation), regardless of the compressibility of the fluid, when there is no flow rate to be measured, a noise signal such as piping vibration or electrical noise is great, and even when this noise signal exceeds a trigger level, it is allowed to be turned into a pulse as a flow rate signal to thereby hinder normal measurement. At this time, when the vortex flow meter is provided with an alarm device, an alarm will be issued.
The above-mentioned problem is especially conspicuous in a flow meter which, as in the case of a vortex flow meter, has a wide flow rate range and detects flow rate through integration based on the flow velocity of the fluid. In a conventional flow rate measurement device, a filter is usually arranged between the flow rate detecting portion in the tubular path and a signal generator (trigger) generating a pulse signal so as to enhance the S/N ratio (signal conditioner); however, since the filter is selected so as to cover the flow rate range, the above-mentioned problem is not solved. This phenomenon constitutes no problem when the cutoff valve is being kept open to pass the compressible fluid to measure the flow rate thereof; in reality, however, the cutoff valve is kept closed almost all day long, so any pulsation of the compressible fluid in the tubular path will be measured as the flow rate. While it is possible to suppress the trigger of such noise to some degree by expanding the trigger level, expansion of the trigger level will result in restriction of the flow rate range allowing measurement.
Japanese Utility Model Examined Publication No. Sho 53-14199 discloses a technique related to an erroneous metering preventing device for preventing erroneous metering in a vortex flow meter. Further, JP 3564111 B discloses a technique related to an erroneous output preventing apparatus (which may also be referred to as erroneous metering preventing apparatus), which is an improvement over the technique as disclosed in Japanese Utility Model Examined Publication No. Sho 53-14199. The techniques disclosed in Japanese Utility Model Examined Publication No. Sho 53-14199 and JP 3564111 B have been proposed by the applicant of the present invention.
In the erroneous metering preventing apparatus disclosed in Japanese Utility Model Examined Publication No. Sho 53-14199, an accumulation counter and a timer are arranged between a signal transmitting portion connected to a flow rate detecting portion in a tubular path and transmitting pulse signals and a metering mechanism for counting the signals to meter flow rate. As a result, when the accumulation counter does not attain a set value within the set time of the timer, it means that an erroneous pulse signal, which has been generated through rocking, is to be off, and no signal is sent to the metering mechanism, which means the metering mechanism is not caused to start integration by rocking of the fluid, thus preventing erroneous measurement. In Japanese Utility Model Examined Publication No. Sho 53-14199, the signal transmitting portion of the erroneous metering preventing apparatus includes a signal conditioner equipped with a filter and a trigger.
In the erroneous metering preventing apparatus disclosed in Japanese Utility Model Examined Publication No. Sho 53-14199, as a result of the observation of the metering condition due to pulsation, attention is focused on the fact that a signal in the metering range is generated in a very short period of time, which signal exhibits a frequency not higher than the lower limit frequency when averaged over a period, for example, of approximately 10 seconds, and, by utilizing this fact, erroneous metering due to fluid rocking is prevented.
The erroneous output preventing apparatus disclosed in JP 3564111 B includes a trigger circuit that outputs pulses when a signal output from a vortex signal detection sensor of the flow rate detecting portion exceeds a predetermined trigger level, an integration circuit for integrating the pulses output from the trigger circuit, two comparators which use a signal obtained through integration by the integration circuit as an input signal and which compare the voltage of the input signal with predetermined upper limit voltage and lower limit voltage to output the comparison results in the form of pulses, a counter circuit for counting the pulses output from the trigger circuit, a logic IC to which the output pulses from the two comparators are input for logical operation and which outputs a signal related to resetting of the count value of the counter circuit, and a pulse output means which does not output the output pulses from the trigger circuit when the count value of the counter circuit is less than a predetermined value but which outputs the output pulses from the trigger circuit when the count value of the counter circuit attains the predetermined value, with a vortex flow meter computing the flow rate or the flow velocity of the fluid to be measured by using the pulses output from the pulse output means.